A reliable and repeatable hole quality is essential, especially in drilling applications using templates. The hole quality depends on the measurement methods, type of hole making apparatus, tool wear and type of material in which the hole is produced.
When the work piece is a stack of sheets of different materials the hole quality depends on how the hole is produced in the different materials and the material characteristics of each sheet in the stack.
When holes are produced in a work piece it is important to identify the orientation and location of the tool in relation to the work piece. When a template is arranged on the work piece the orientation of the tool in relation to the work piece is given, but for example when producing a hole provided with a countersink the depth of the countersink is critical in order to achieve an interacting connection between a fastening element and the hole provided with the countersink.
When a number of holes of a predetermined accuracy are to be made, different methods and systems are known for identifying each individual hole, collecting and registration information about the work piece and the tool when the holes are produced in the work piece, and also for measuring the holes after they have been produced in the work piece. Often such measuring methods are based on a sample of the produced holes and therefore some of the produced holes may not fulfil the high demands of accuracy.
Measuring devices and methods for measuring the position of a tool in relation to a work piece are known. Document U.S. Pat. No. 5,181,809 discloses a device and a system enabling a tool to be brought to a precise location of a drilling template. However, the device is not arranged to measure the distance between the work piece and the drilling template and therefore this device is not suitable when producing holes provided with for example a countersink.
Different types of hole making apparatuses are available. For special applications such as hole making in the aircraft structure of an aircraft the demands of accuracy are extremely high and therefore special hole making apparatuses should preferably be provided. Such a special hole making apparatus may use the orbital drilling technique. Orbital drilling is based on machining the material both axially and radially by rotating the cutting tool about its own axis as well as eccentrically about a principal axis while feeding the cutting tool through the material. The general principles in orbital drilling are for instance disclosed in U.S. Pat. No. 5,641,252 and EP-B1-1102653. Other types of hole making apparatuses for making holes of extremely high accuracy are also possible to use.